The present invention relates to paper machines and in particular to a press section for dewatering a web which may be a suitable paper, cardboard, or the equivalent thereof, this press section having at least three consecutive press nips defined by suitable rolls while including a common endless fabric means in the form of a suitable felt or wire which travels through all of the press nips with the web adhering to this common fabric means so as to be conveyed thereby through the three press nips. On the other hand, at least at the first two press nips the web is engaged by an additional fabric means so that a double-felt action is provided at the first two press nips.
In the manufacture of certain types of paper, cardboard, or equivalent products, it is desirable to carry out the dewatering in such a way that the original web dimension, its length among others, is maintained as nearly unchanged as possible.
When manufacturing paper to be used in bags, for example, the paper web formed on a wire section is conveyed through the press section in such a way that an attempt is made to prevent the web from being subjected in the press section to stresses which might result in elongation of the web. The same considerations apply to the manufacture of stretchable types of paper in those instances where the web has been mechanically contracted or shortened prior to the press section.
On the other hand, the press section should be constructed in such a way that when the web travels beyond the press section the web has as uniform a moisture content as possible both in the longitudinal machine direction of the web as well as in the transverse cross-machine direction. Such a requirement with respect to the uniformity of the moisture is encountered, for example, in the manufacture of bag paper, particularly of so-called wet-upset types of paper which are conveyed through a multiple cylinder drying section while applying to the web an absolute minimum of tension as by utilizing so-called slack runs. The same requirement is encountered in the event that the web is treated in a half-wet state in one way or another as by being worked, for example, with press or micro-creping means.
The construction and operation of the press section also influences the quality of the web, for example the quality of the surface of the paper web. If it is possible to carry out the dewatering of the web for the most part by pressing the web in between felts, then it is possible to avoid an increase in the smoothness of the paper surfaces, and such a lack of smoothness is important, for example, in the case of bag paper. In fact, all desired surface characteristics on both surfaces of the web may be efficiently influenced by way of the quality of the felts.